Observation of algebraic time order for two-dimensional dipolar excitons

We study dipolar excitons confined in a flat trap of a GaAs bilayer heterostructure. By quantifying density and density fluctuations at sub-Kelvin temperatures, we unveil the excitons' universal equation of state. This allows us to infer thermodynamically the critical phase-space density for the transition expected between normal and superfluid regimes. The region of this crossover is directly confirmed by a net change in the excitons' temporal coherence, from an exponential to an algebraic decay with an exponent compatible with the Berezinskii-Kosterlitz-Thouless theory.